A structure formed by coupling two plate materials by caulking will be described using FIG. 5. In the drawing, a plate-shaped lower member 101 is preliminarily provided with a through-hole 103. A stepped and bottomed hole 107 coaxial with the through-hole 103 in the lower member 101 is provided to an upper surface of a plate-shaped upper member 105 laminated on the lower member 101 by an upper punch. The stepped and bottomed hole 107 is formed of a small diameter portion 107a on a bottom side and a large diameter portion 107b on an opening side.
Because a diameter of the upper punch is larger than a diameter of the through-hole 103, a rim portion of the through-hole 103 in the lower member 101 undergoes deformation and a cone portion 109 becoming smaller in diameter toward a lower surface is formed along the rim portion of the through-hole 103 in the lower member 101. Further, because the cone portion 109 is formed, the through-whole 103 has become a tapered through-hole becoming larger in diameter toward the lower surface.
The upper member 105 is provided with a protrusion 111 that fits into the cone portion 109 and the through-hole 103 in the lower member 101 and is in press-contact with an inner wall surface 109a of the cone portion 109 and an inner wall surface 103a of the through-hole 103.
Further, a conical recess 113 is provided to a lower end face of the protrusion 111 of the upper member 105. By providing the conical recess 113, flesh of the protrusion 111 of the lower member 101 that has fit into the through-hole 103 is guided to the inner wall surface 103a of the through-hole 103 and an outer wall surface 111a of the protrusion 111 comes in close contact with the inner wall surface 103a of the through-hole 103 (see, for example, Patent Document 1).